The present invention relates to a method to monitor a process for treatment of vegetable or animal oils such as refining, bleaching or deodorization, in which process a number of process steps are carried through under vacuum created by a vacuum system. The invention also relates to an arrangement to carry through the proposed method.
When producing vegetable or animal oils from different raw materials there are usually a number of process steps, where the oil is treated under vacuum with or without stripping gas. The aim of these process steps is to remove substances that impart disagreeable odours and taste. These substances may have been formed during different earlier treatment steps or may have been present already in the cruel oil. The result of this vacuum treatment is to a high degree dependent on really obtaining the desired vacuum level. The vessels in which the vacuum treatment takes place are usually of imposing size. A large plant may for example have a vessel with a height of 35 meters and a diameter of 3 meter. Usually, the vacuum vessels are provided with vacuum meters to control the vacuum.
Often a number of vacuum vessels, in which the oil is treated, are connected to a common vacuum source. This may consist of a vacuum system comprising vacuum generating means and condensers. The vacuum generating means may consist of a vacuum pump or of a steam ejector. In large plants a number of steam ejectors with intermediate condensers are often used. The condenser is chilled with water, distillate or some other suitable liquid in order to achieve condensing of condensable gases.
In EP 513 739 there is shown a deodorization system with vacuum boosters and condensers, which equipment makes it possible to reach the high vacuum that is necessary for carrying through the treatment. According to this prior art the non-condensable gases are discharged to the atmosphere.
The vacuum system operates to achieve a vacuum level of 0,5-6 mbar in the different vessels and heat exchangers used during the treatment of the fatty oil. The added stripping gas as well as dissolved gases in the oil and various compounds formed during the treatment are removed from the treatment vessels.
An efficient treatment of the fatty oils requires the mentioned vacuum levels. The vacuum value is usually monitored by a pressure sensor. Working at this high vacuum means that the instrument used for sensing the pressure firstly must be evacuated to absolute vacuum and that the pressure in the vessel is then measured in relation to absolute vacuum.
Some leakage in the equipment or in the pumps or valves, which are necessary to build up the plant, will result in problems to obtain the desired vacuum value. Air leaking into the vessel will also cause undesired oxidation and may lead to spoiled products.
One aim of the present invention is to bring about an accurate and rapid control of the vacuum value in the equipment, another to make it possible to obtain an early detection of any faults in the vacuum system. The present invention also means that start-up of a plant may be done in a much shorter time than according to prior art.
According to the method of the present invention it is now proposed that the mass flow of non-condensable gas out from the vacuum system is controlled and registered. During steady-state operation in a tight plant the mass flow through the mass flow meter will be constant. The stripping gas is added in a known and constant amount. If the stripping gas consists of steam it is condensed in the condensers in the vacuum system. If some inert gas is used for stripping the flow of non-condensable gas will be greater but the value will be a constant one if the equipment functions.
An arrangement in a plant for carrying through the method according to the invention comprises a vacuum system with at least one vacuum creating means and at least one condenser and a mass flow meter for gas, which by way of a pump is connected to the last condenser in the vacuum system. Mass flow meters of a suitable kind are available on the market.
According to the method of the invention it is possible to control the amount of gas which passes the mass flow meter all the time. During operation conditions the mass flow is constant and represents the content of non-condensable gases in the stripping gas. Should this value be changed but the pressure remain constant there is a leakage somewhere in the plant.
When the plant is new or shall be started again after a stop the vessels and other equipment put under vacuum is evacuated, after which the vacuum system is cut off. If there is an increase in pressure there is a leakage in the plant. Often one starts by controlling each separate vacuum vessel and thereafter the whole plant. This is circumstantial and may demand from some days up to several weeks in leak searching if the tests must be redone.
According to the method of the invention the evacuation process may be controlled and if the pressure (in relation to the mass flow) in the equipment is not lowered at the rate, which may be expected knowing the capacity of the vacuum system, one obtains an early indication that there is a leakage somewhere or there is something wrong with the vacuum system.
The proposed method gives alarm far more early than the generally used method of monitoring only the pressure.